The present invention relates to a tire with radial carcass reinforcement which is intended to bear heavy loads, and more particularly to a xe2x80x9cheavy-vehiclexe2x80x9d-type tire, intended to be fitted on vehicles such as, for example, lorries, road tractors, buses, trailers and others, and more particularly to the novel reinforcement structure for the beads of said tire.
Generally, a tire of the type in question comprises a carcass reinforcement formed of at least one ply of metal cables, which is anchored in each bead to at least one bead wire, forming an upturn. The carcass reinforcement is radially surmounted by a crown reinforcement, composed of at least two plies of metal cables which are crossed from one ply to the next, forming angles of between 10xc2x0 and 45xc2x0 with the circumferential direction. The carcass reinforcement upturns are generally reinforced by at least one bead reinforcement ply formed of metal cables which are oriented at a small angle relative to the circumferential direction, generally of between 10xc2x0 and 30xc2x0.
In the case of the presence of a single bead reinforcement ply, the latter is generally located axially to the outside, along the carcass reinforcement upturn, with a radially upper end located above or below the radially upper end of the carcass reinforcement upturn. As for the radially lower end of such a reinforcement ply, it is generally located on a straight line parallel to the axis of rotation and passing approximately through the center of gravity of the meridian section of the anchoring bead wire of the carcass reinforcement.
The known solution aims to avoid deradialization of the cables of the carcass reinforcement upturn and to minimize the radial and circumferential deformations to which the end of said upturn and the outer rubber layer covering the bead and providing the connection to the rim are subjected.
The life of xe2x80x9cheavy-vehiclexe2x80x9d tires, owing to the progress achieved, and to the fact that certain types of travel are made less of a handicap as far as wear of the tread is concerned, has become such that it is also necessary to improve the endurance of the beads. Said improvement must focus on the degradation of the rubber layers at the level of the ends of the carcass reinforcement upturn and the radially outer ends of the bead reinforcement plies. More particularly in the case of tires which are subject to prolonged travel, which travel frequently induces a high temperature of the beads, owing to the temperatures which the mounting rims reach, the rubber mixes in contact with the rim are then subject to a reduction in their rigidity, and to more or less slow oxidation, hence the very marked tendency of the carcass reinforcement to unwind from around the bead wire under the action of the internal inflation pressure, despite the presence of one or more bead reinforcement plies. There then arise bead wire movements and shearing deformations at all the ends of the plies, resulting in the destruction of the bead. Said improvement must also, and primarily, focus on this second possibility of degradation.
The carcass reinforcement of a radial tire, mounted on its operating rim and inflated to the recommended pressure, has in one sidewall a regularly convex meridian profile between approximately the zones of connection with the meridian profile of the crown reinforcement and with the bead. In particular, starting from the radius where the carcass reinforcement is subject to the influence of the bead reinforcement ply (plies), said reinforcement has a meridian profile which is substantially rectilinear or even curved in the opposite direction to the curvature in the sidewalls, that is to say, substantially parallel to the curvature of the rim flanges starting from a point of inflection located radially approximately at the level of the radially upper end of the bead reinforcement ply which is placed along the carcass reinforcement upturn. Such an architecture results in relatively thick tire beads.
The problems of endurance referred to above, which are influenced by the operating temperature of the beads, have been solved to a very slight degree by a highly reinforced bead structure. French Patent 2 654 988 describes a tire, particularly suitable for heavy vehicles, having a radial carcass reinforcement which is formed of inextensible reinforcement elements and anchored within each bead. Said bead is reinforced by two plies of steel cables, a first ply turned up about the bead wire to form two strands, and a second ply arranged axially to the inside of the main part of the carcass reinforcement. Said problems can also be solved by what is called a xe2x80x9cthinnedxe2x80x9d bead structure, obtained, for example, by the means described in French Patent 2 415 016 which, to this end, teaches to impart a relatively great concavity to the outer sidewall of the tire, between the point where said sidewall is no longer in contact with the rim and the point where said sidewall achieves its maximum distance relative to the equatorial plane, when the tire is mounted on its operating rim and inflated to the recommended pressure.
Such an arrangement becomes insufficient, in particular when the ratio of the height H on the rim to the maximum axial width S of the tire decreases, and requires improvement when the loads borne become greater, or the inflation pressures less, than the recommended loads and pressures.
Research has led to the conclusion that the thinning of the beads had to be effected not from the outside of the tire but from the inside, and that it needed to be effected in combination with specific reinforcement characteristics of said beads.
In order to improve the endurance of the beads of a tire, for a vehicle intended to bear heavy loads, said tire, according to the invention, comprises at least one radial carcass reinforcement which is formed of at least one ply of inextensible reinforcement elements and is anchored within each bead B to at least one bead wire to form an upturn, each bead B being reinforced by an additional reinforcement armature formed of metal elements. It is characterized in that, viewed in meridian section, the carcass reinforcement has a meridian profile, the trace of which between the point A of greatest axial width and the first point of tangency T with the virtually circular coating layer of the anchoring bead wire, is convex over its entire length, and such that the thickness xcex5 of the bead B, measured on a line perpendicular to said meridian profile at a point C of height hc of between 30 and 40 mm and measured relative to the base of the bead YYxe2x80x2, is between 2.5 and 3.0 times the thickness e of the sidewall measured on the straight line Dxe2x80x3, which is the line of greatest axial width, said carcass reinforcement being reinforced, in its non-upturned part, by the additional reinforcement armature formed of at least one ply of circumferential reinforcement elements which is arranged axially to the inside of said non-upturned part.
The point A of greatest axial width is, by definition, the point of the meridian profile of the carcass reinforcement corresponding to the point of greatest axial width of the carcass reinforcement when the tire is mounted on its operating rim, non-loaded and inflated to the recommended pressure, and the straight line Dxe2x80x3 of greatest axial width is the straight line parallel to the axis of rotation and passing through A.
The elements of the additional reinforcement armature are said to be circumferential if the angle which they form with the circumferential direction is between +5xc2x0 and xe2x88x925xc2x0.
The convex trace of the meridian profile of the carcass reinforcement is generally formed of a plurality of adjacent arcs of a circle, the radii of curvature of which increase from the point A of greatest axial width to the point of tangency T with the virtually circular coating layer of the anchoring bead wire, but the trace between the point A and the point T may be convex and circular with a center of curvature located on the straight line of greatest axial width.
The ply or plies of the additional reinforcement armature, axially to the inside of the non-upturned part of the carcass reinforcement, will advantageously have its (their) radially lower end(s) radially below the straight line Dxe2x80x2, parallel to the axis of rotation and passing through that point of the coating layer of the anchoring bead wire which is radially farthest from the axis of rotation, but above the straight line D which is parallel to the axis of rotation and passes through that point of the coating layer of the anchoring bead wire which is radially closest to the axis of rotation, and radially upper ends located at a radial distance from the straight line D between a quantity equal to half the radial distance between the straight lines D and Dxe2x80x3, increased by half the radial distance between the straight lines D and Dxe2x80x2, and a quantity equal to half the radial distance between the straight lines D and Dxe2x80x3, reduced by half the radial distance between the straight lines D and Dxe2x80x2.
This structure thus permits partial takeup of the meridian tensile forces to which the carcass reinforcement is subjected, and thus minimizes the radial deformations at the ends of the carcass reinforcement upturn, whatever the conditions of travel.
The takeup of the forces will be all the greater the greater the number of reinforcement elements of the additional armature; they will preferably be metal, made of steel, which are parallel to each other within the additional ply and adjoining over the entire width of the reinforcement armature. Reinforcement elements are said to be adjoining if, in the direction perpendicular to said elements, the distance between two adjacent elements is reduced as much as possible.
The takeup of the meridian tensile forces of a carcass reinforcement is known per se from numerous documents. French Patent 750 726 teaches gluing carcass reinforcement plies to auxiliary reinforcements, each formed of a cord wound in a spiral and consequently having significant flexibility in the transverse direction. Said gluing is effected such that two carcass reinforcement plies tightly surround an auxiliary reinforcement, this surrounding being in the absence of a bead wire and a carcass reinforcement upturn.
French Patent 1 169 474 also teaches dispensing with the bead wire usually used by small plies of metal cords or cables inclined on the parallel hooking line, the angle of inclination possibly being as low as 5xc2x0, the carcass reinforcement ply (plies) being turned up or not turned up.
French Patent 1 234 995 provides for the replacement of the usually oblique reinforcement elements of the bead reinforcement ply (plies) which is (are) intended to reduce the movements of hooking against the rim flange with circumferential elements, the reinforcement armature possibly being arranged between the main part of the carcass reinforcement and the upturn of said reinforcement, or axially to the outside of said carcass reinforcement upturn.
The same bead reinforcement armatures with circumferential elements are found in French Patent 1 256 432, said elements being intended to take up the tensile forces of the carcass reinforcement in their entirety, and in the case of tires for passenger vehicles, in the absence of anchoring bead wires or any other reinforcement ply.
FR 2 055 988 describes a certain number of possible applications of a bead reinforcement armature having circumferential elements, in particular the use of such an armature in the place of the conventional armature having oblique elements which are arranged axially to the outside of the carcass reinforcement upturn.
Not only does the combination of the meridian profile according to the invention with the presence of a bead reinforcement armature, having circumferential elements, and at least one ply of which is arranged within the non-upturned part or main part of the carcass reinforcement, permit improvement of the endurance of the beads, but it also makes it possible to obtain a very substantial reduction in the weight of a tire while not adversely affecting its properties, when the presence of a bead reinforcement armature described above is combined with replacing the bead wire having rectangular metal wires which is generally used in the type of tire in question with a more economical bead wire of the braided type, that is to say, one formed of helical braiding of several metal wires, or a bead wire of the type having a stack of polygonal section, that is to say, a bead wire obtained by winding a metal wire on a form, resulting in several layers of different widths. The types of bead wire above mean that less space is taken up, owing to the form of their cross-section, but also owing to the fact that the takeup of the forces by the additional armature permits a substantial reduction in said sections. Whatever the type of bead wire, whether braided or in a stack, the cross-section may be reduced by 30 to 50% which, relative to the invention being applied to the case of a bead wire having rectangular wires, which takes up more space, results in having either a thinner thickness of bead, or a thickness of bead which is constant but has a meridian profile which is said to be flatter, in the sense that its convexity is less pronounced.
The bead reinforcement armature thus described may be produced separately by spiral winding on a horizontal support of suitable shape, and then be transferred to the non-vulcanized tire blank. The manufacture of such a tire will however be advantageously facilitated by the use as reinforcement elements of lengths or assemblies or groups of lengths of cables of a circumferential length of between 0.2 and 0.4 times the average circumferential length of the reinforcement armature, which permits laying of the bead reinforcement armature on the building drum for the raw blank of the carcass reinforcement and the shaping of said blank into a torus without major difficulties, and said average length being measured upon laying on said building drum. The circumferential gaps or cuts between cut elements will preferably be offset from each other.
It may also be advantageous, in order to impart greater transverse rigidity to the tire bead, while permitting greater improvement of the endurance of the beads, for at least one ply formed of circumferential metal elements and arranged axially to the outside of the non-upturned part of the carcass reinforcement to be present in the additional reinforcement armature. Said additional ply, axially to the outside, will have its radially upper end located radially in the same manner as the ply or plies axially to the inside, whereas the radially lower end of this second ply will advantageously be located between the straight line Dxe2x80x2 and a straight line parallel to Dxe2x80x2 radially to the outside and radially distant from Dxe2x80x2 by an amount equal to the radial distance between Dxe2x80x2 and D. The reinforcement elements of said second ply will preferably be lengths or assemblies of lengths of metal cables made of steel, of a circumferential length at most equal to 0.4 times the average circumferential length of the reinforcement armature thus formed.